1. Field of the Invention
The present invention relates generally to a weft-bar (set mark) prevention system for a loom and more specifically to a system for preventing the heavy or light weft (filling) bar caused when a loom is restarted, after the loom has been stopped due to weft or warp thread cut (weft or warp thread breakage).
2. Description of Prior Art
In a loom, as is well-known, warp threads let-off from warp beams are guided and introduced to the cloth fell under an appropirate tension applied thereto by the aid of an elastically supported guide member, for instance, such as a back roller. In such a loom, however, if the loom is stopped due to some reasons such as warp cut or weft cut (i.e., breakage of the warp or weft thread) and if it takes a time before starting the loom again, since the warp threads are left as they are under tension, the warp threads are stretched and become somewhat elongated.
If the warp threads are elongated, since the cloth fell moves frontward (toward the breast beam side), when the loom is restarted, there exists light weft-bar in the initial several picks. A weft-bar is also known as a set mark.
Additionally, when the loom is restarted, the initial several picking motions (the first picking motion, in particular) are performed while the speed of the main motor is increasing, the speed of the reed is lower than in the stable state. Furthermore, when the loom is restarted, since the frontward deflection (deformation) of the reed caused by the reed inertia and reed elasticity when the reed beats the weft is smaller than in the stable state, the reed is positioned more rearward than in the stable state at the beat-up stage, thus causing a light weft-bar when the loom is restarted.
Two overcome this problem, there has been proposed a method of preventing the light weft-bar which involves intentionally increasing the tension applied to the warp threads by tension increasing means during the initial several picks after the loom has been restarted, in order to adjust the position of cloth fell.
By way of background, the loom is stopped when a warp thread is cut or a weft thread is not inserted correctly. In the case of warp thread cut, the loom is usually repaired in the state where the warp threads are closed (in a closed-shed state); in the case of weft thread cut, the loom is usually repaired in the state where the warp threads are opened (in an open-shed state). Therefore, when the loom is restarted beginning from the motion angle at which the loom has just been repaired, the loom-restarting angle in the case of warp thread cut is different from that in the case of weft thread cut.
On the other hand, since the initial revolution speed of the reed drive shaft at the beat-up stage increasingly varies at the first picking motion according to the loom-restating angle, the degree of the reed deflection differs. Therefore, in the case where a tension to be applied to the warp threads is predetermined to be an appropriate value required when the loom is restarted beginning from the closed-shed state, the weft bar tends to be heavy when the loom is restarted beginning from the open-shed state (the first pick motion is attained after one empty beat-up motion). In the case where a tension to be applied to the warp threads is predetermined to be an appropriate value required when the loom is restarted beginning from the open-shed state, the weft bar tends to be light when the loom is restarted beginning from the closed-shed state.
In practice, accordingly, it is necessary to restart the loom beginning from the same predetermined loom-restarting angle. As a result, when the loom has been repaired at an angle different from this predetermined angle, it is necessary to move the loom manually to this predetermined angle before restarting, thus complicating the procedure.